For urging flow of and/or for pressurizing fluids, pumps are available in a large variety of configurations, most of which are specific for their respective applications. One general group of pumps used particularly in certain fluid-dispensing applications is “metering pumps,” which are configured for moving precise volumes of fluid accurately in specified time periods. Examples of metering pumps include piston pumps, syringe pumps, diaphragm pumps, bellows pumps, and peristaltic pumps. Because piston pumps are generally positive-displacement, even against substantial back-pressure, they are very effective for performing accurate delivery of many types of liquids.
Most types of piston pumps and syringe pumps (the latter actually being a subset of piston pumps) include at least one piston that urges fluid flow by undergoing a series of paired, alternating linear strokes. Each pair of strokes includes an intake stroke and a discharge stroke. The piston extends through a dynamic seal into a housing. During the intake (suction) stroke, the piston is pulled or otherwise moved relative to the housing so as to draw fluid into the housing via an inlet port. During the discharge stroke, the piston is pushed or otherwise moved relative to the housing so as to displace fluid from the housing via an outlet port. The inlet port and outlet port usually are controlled by respective valves that open and close at appropriate moments to control fluid movement into and out of the pump during the respective strokes. (The valves are not necessarily located immediately at the inlet and outlet ports.) In piston pumps in which the piston undergoes reciprocating linear motion relative to the housing, the piston can be actuated by any of various mechanical means or electromechanical means (e.g., motor-and-gear mechanisms or solenoid mechanisms, respectively).
Whereas metering pumps that include discrete inlet and outlet valves are satisfactory for many applications, problems become manifest when such pumps are used in certain other applications. Exemplary problematic applications are the pumping of viscous liquids and thick liquid suspensions such as food ingredients and certain industrial liquids such as liquid adhesives, resins, paints, concentrates, and the like. Many viscous liquids and liquid suspensions interfere with proper functioning of valve seals and valve seats, especially over time, which can degrade the desired positive-displacement pumping action as well as pumping accuracy and precision. Other disadvantages, especially with pumping of liquid food substances and other sanitary liquids, are the ease with which valves become contaminated and the inherent difficulty of cleaning and disinfecting valve mechanisms to ensure consistently hygienic pumping action.
To address the valve problem summarized above, so-called “valveless piston” pumps have been developed that effectively eliminate inlet and outlet valves by incorporating valving action in the motion of the piston. A conventional valveless piston pump 200, shown in FIG. 6, comprises a piston 202, a piston housing 204, a dynamic seal 206, a motor 208 (with armature 210 and shaft 212), and a rotational coupling 214. The piston housing 204 comprises an inlet port 216 and an outlet port 218. The piston 202 is cylindrical, extends along a piston axis Ap, and slip-fits into a bore 220 defined in the housing 204 (or in a liner 205 situated in the housing, as shown). The piston 202 comprises a proximal end 222 and a distal end 224. The distal end 224 has a flat 226 or analogous cutout that extends part-way around the circumference of the distal end 224 and is situated inside the bore 220 during operation. The proximal end 222 comprises a pin 228 extending substantially perpendicularly to the piston axis Ap.
Even though the piston 202 slip-fits into the bore 220, the dynamic seal 206 is required because the slip fit does not isolate the bore from the external environment sufficiently to prevent leaks and troublesome accumulation of dried or congealed fluid. The dynamic seal 206 forms a sliding seal circumferentially around the piston 202 in a region of the piston between the flat 226 and the proximal end 222, and allows both reciprocating motion (along the piston axis Ap; arrow 225) and rotational motion (about the piston axis Ap; arrow 227) of the piston in and relative to the bore 220.
The rotational coupling 214 comprises a proximal end 230 and a distal end 232 arranged at substantially right angles to each other. The distal end 232 comprises a spherical bearing 234 that receives the pin 228 and allows rotation of the pin relative to the coupling 214. The proximal end 230 of the coupling 214 is attached to the shaft 212 of the motor armature 210 so as to undergo rotation about the motor axis Am whenever the armature is rotating. Energization of the motor 208 causes rotation of the armature 210.
As noted above, during operation the piston 202 undergoes both rotational and reciprocating motion in the bore 220. The rotational motion is a direct result of rotation of the motor armature 210. To achieve the accompanying reciprocating motion the piston axis Ap is angled (at an appropriate “obtuse” angle, i.e., greater than 90° but less than 180°) relative to the motor axis Am. Thus, as the armature 210 rotates about the motor axis Am, the piston 202 undergoes synchronous rotation and reciprocation in the bore 220.
The particular configuration of the distal end 224 of the piston 202 serves two functions. First, in the bore 220 the flat 226 defines a pathway for fluid being aspirated into the bore via the inlet port 216 and a pathway for fluid being discharged from the bore via the outlet port 218 as the piston 202 undergoes reciprocating motion. Second, as the piston 202 is being rotated in the bore 220 about the piston axis Ap, the remaining (not flatted) portion of the distal end 224 periodically opens and closes the inlet port 216 and the outlet port 218 in a synchronous manner relative to the reciprocating motion of the piston. Thus, the inlet port 216 is opened (and the outlet port 218 is closed) during a time increment in which the piston 202 can aspirate fluid into the bore 220 via the inlet port, and the inlet port 216 is closed (and the outlet port 218 is opened) during a subsequent time increment in which the piston 202 expels fluid from the bore via the outlet port.
The length of the “stroke” undergone by the piston 202 in the bore 220 is determined by the obtuse angle of the piston axis Ap relative to the motor axis Am. Within a defined range, the smaller the angle, the longer the stroke and the greater the pumping rate exhibited by the pump 200 at a given reciprocation rate. The stroke is zero at an angle of 180° (i.e., when the axes Ap, Am are parallel to each other) and is at a functional maximum at an angle of about 135° to 150°. Angles less than about 135° impart a stroke that is too long. I.e., an excessively long stroke results in the piston 202 being pulled too much out of the bore 220, which causes the piston 202 to open both the inlet port 216 and the outlet port 218 simultaneously and thus stop pumping action (which requires the synchronous alternating opening and closing of the ports relative to the reciprocating motion of the piston). Also, an excessively long stroke applies excessive strain to the dynamic seal 206 and the spherical bearing 234.
Conventional valveless piston pumps as summarized above are effective metering pumps for many uses, particularly in view of their lack of valves and their ability to achieve positive-displacement pumping even of viscous liquids. Unfortunately, however, conventional valveless piston pumps are problematic when used for certain other applications. The main reason for this shortcoming is the dynamic seal 206, which is prone to leaks, tends to harbor contamination, and is difficult and time-consuming to clean (which frequently must be performed in situ). The dynamic seal 206 also inherently has low reliability and thus requires frequent servicing or replacement relative to other parts of the pump 200. These disadvantages are particularly important in valveless piston pumps being considered for use in food- and medicament-dispensing applications.
Therefore, there is a need for valveless piston pumps that do not have a dynamic seal.